DisplayPort (DP) is a display interface standard of the Video Electronics Standards Association (VESA), Newark, Calif. 94560. See DisplayPort Specification, Version 1.3, September 2014. DisplayPort allows video and audio to be coupled from a computer to a video display or an audio playback system. A DisplayPort connector supports 1, 2, or 4 data pairs in a Main Link that also carries clock and optional audio signals with symbol rates of 1.62, 2.7, or 5.4 gigabits per second. The Embedded DisplayPort (eDP) standard builds on the DP standard.
Display or sink devices can be connected to source devices, such as personal computers or consumer electronic devices, either directly or through what are called branch devices. Between the source and the sink devices, there may be one or more branch devices. And between any two of these devices, there may be one or more PHY level repeaters (referred to herein as retimers).
Some source device components (e.g., SoCs) have significantly more features than just the DP transmitter (TX) functionality. Due to schedule and effort reasons, it is often the case that difficult decisions are made about exclusion of certain optional features. At times, those excluded features become more valuable due to other developments later. It is also possible that a new feature or two get introduced in the industry specification in response to interop issues or market demands. These in turn create a need for (even full featured) Source device components that are already in the market to somehow be upgraded by original equipment manufacturers (OEMs) for their upcoming platforms to include these new features. Features that do not require any hardware changes can be quickly turned around using software revisions. However, if hardware changes are required, a large amount of time is needed to design and manufacture a new product with the hardware changes.
The above reasoning for feature enhancements also apply to DP TX components in the branch devices. There are some additional reasons also. First, branch devices may have functional limitations since they are developed independently of source devices, and potentially (deliberately) targeting a specific subset of the optional DP features in the interest of cost, schedule, or power. OEMs may also be interested in using legacy branch devices with newer source devices for cost, supply, robustness, or any number of other practical reasons. This could limit the end-to-end capability from a source to a sink even though both the source and sink devices may be individually capable of a specific feature.
One way to address this problem is to create a new branch device with this feature. Such a device could be inserted after a deficient source or a deficient branch to address the capability deficiency. While less complex than a source device, branch devices are complex in their own right when compared to PHY-level repeaters: they implement functions above the PHY layer (including topology, link, and session management functions), and they usually implement MST functionality in order to not limit MST functionality downstream from the branch device. These add up to branch devices being higher power and higher cost. Lastly, a DP branch device is an addressable entity; this could be a cause for concern in some topologies that already features significant number of branch devices because the DP specification has a limit on the number of devices on a given path from a source device to a sink device.
DP's PHY repeaters (LT-Tunable PHY layer repeaters) are lightweight, low cost, low power devices that are integrated within a source device or a branch device. They are designed to serve a single function, namely enhancing electrical characteristics of a link that may have deteriorated due to connectors or long routes.